This note focuses on the robust performance control design problem in single-loop systems from the viewpoint of the Quantitative Feedback Theory (QFT) method. Unlike the more general setup for QFT which allows arbitrary plant uncertainty description (i.e., value sets or templates), we assume that the plant uncertainty is described by multiplicative perturbation. A set of quadratic inequalities is developed for the robust performance problem in general, and, in particular, for the robust complimentary sensitivity problem. The so-called QFT bounds can be computed in a closed-form from these quadratic inequalities.

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